I don’t know how to spell ambience, which is unfortunate because it comes up a lot in the articles I edit. Perhaps half the articles I edit that use this word write it “ambiance,” and the other half spell it “ambience.” It’s my job to know which one is right. Until recently, I thought I did.
I’ve looked it up quite a few times. When I’m editing for a newspaper, I usually check the online version of “Webster’s New World College Dictionary.” That’s the “Associated Press Stylebook’s” “designated Webster’s” — the source for all matters not covered in the style book itself.
For example, for the choice between “health care” and “healthcare,” AP lists its advice right under the letter H. Go for the two-word version, AP’s 1992 edition says. But AP has no listing for “child care” or “childcare.” So you have to check “Webster’s New World,” which has no listing for the one-word “childcare.” You form the term yourself using words that are in the dictionary: “child care.” (This Webster’s does have a listing for “child-care,” but only as an adjective: child-care center.)